1. Field of the Invention
The present invention relates to a radar apparatus having the function of estimating a target's width.
2. Description of the Related Art
An FM-CW radar equipped with a function to scan the radiowave projection direction mechanically or electronically, for example, can not only measure the distance and relative velocity of a target but also determine the bearing to the target (azimuth angle to the target). Further, as described, for example, in Japanese Unexamined Patent Publication No. H11-183601, the lateral position and width of the target can be determined by analyzing the distribution of reflections from the same target with respect to the projection angle. If the lateral position and width of the target can be determined, vehicle-to-vehicle distance control can be performed, for example, in such a manner that, when a vehicle traveling ahead moves into the same lane as the radar-equipped vehicle, the vehicle speed is reduced to maintain a prescribed vehicle-to-vehicle distance and, when the vehicle traveling ahead moves into another lane, the speed is increased up to the preset cruising speed.
The lateral position X and width W of a target obtained by analyzing the distribution of reflections from the same target with respect to the projection angle θ can be expresses asX≈R sin θm  (1)W≈R tan Δθ  (2)where θm is the angle corresponding to the center of the distribution, Δθ is the angle difference corresponding to the width of the distribution, and R is the distance to the target; here, it is understood that Δθ<<0.
As can be seen from the equation (2), the angle difference Δθ decreases as the distance R to the target of width W increases. When Δθ decreases to a value almost equal to the scanning step width of the projection angle, only one or two reflections can be obtained from the same target, and as a result, the width W of the target cannot be determined. Therefore, in this case, an estimated value is used for the width W of the target. At this time, error (uncertainty) in the lateral position X increases, because the angle θm corresponding to the lateral position of the target cannot be determined with finer resolution than the step width of the projection angle.
If, for example, a predetermined given value is used as an estimated value for W, and as the error in the lateral position X (azimuth angle θm) is large, there occurs a problem such that, when the vehicle traveling ahead is not actually in the same lane as the radar-equipped vehicle, control is performed for deceleration by erroneously determining that the vehicle ahead has moved into the same lane.